A method of this type is described in German Patent No. 19513 614. According to this patent, the coating of a substrate with amorphous layers of carbon takes place through the application to the substrate of a bipolar voltage whose positive and negative pulse durations can be adjusted separately from one another. During the depositing, the positive pulse duration is smaller than the negative pulse duration, and the pulse frequency is in the range from 5 to 100 kHz. In order to improve the adhesion of the produced amorphous carbon layer to the substrate, a modified carbon intermediate layer is applied that contains metal. In this conventional method, plasma production and ion bombardment of the growing layer are realized together using the applied bipolar voltage, and cannot be controlled individually. For many qualities of layer, the layer deposition using this process is therefore limited to a comparatively narrow process window.
From the German Patent No. 196 09 804, a method is described for the plasma coating of bulk material in which a rotating basket moves slowly about a plasma coating source. A voltage can be applied to the rotating basket in order to bring the bulk material to be coated to an electrically negative potential. Means for producing a cleaning plasma, with which the bulk material is cleaned before the beginning of the layer deposition, are located inside the rotating basket. In this context, the cleaning plasma is produced independently of the voltage applied to the rotating basket and to the bulk material. A negative electrical charging of the bulk material to be coated is in general also provided for the subsequent coating step. No further indications are disclosed concerning the manner in which the negative charging is to take place.
A method for the manufacture of hard amorphous carbon layers is described in R. S. Bonetti, M. Tobler, Aindustriell hergestellte diamantartige Schichten,@ Oberfläche und JOT, vol. 9, 1988, p. 15. In a plasma-supported CVD method, plasma production and negative substrate bias voltage are realized together, using a radio-frequency (RF) power supply applied to the substrate. The substrate potential ensures the ion bombardment required for the depositing of layers that are dense and hard and therefore resistant to wear. For this purpose, the ratio between the surface of the parts to be coated and the inner wall surface of the recipient must be smaller than 1, which limits, in an undesirable manner, the charge density and ability to scale the method upward for industrial charge quantities. Another disadvantage is the required load-dependent matching of the RF coupling.